- Plant kingdom is divided into Algae, Bryophytes, Pteridophytes, Gymnosperms, and Angiosperms.
- Artificial system of classification used gross superficial morphological characters and were based mainly on vegetative characters or on the androecium structure.
- Natural classification systems were based on natural affinities among the organisms and consider not only the external features, but also internal features.
- Phylogenetic classification systems are based on evolutionary relationships between various organisms which assumes that organisms belonging to the same taxa have a common ancestor.
- Numerical Taxonomy is based on all observable characteristics, where number and codes are assigned to all the characters and the data are then processed.
- Cytotaxonomy that is based on cytological information like chromosome number, structure, behavior.
- Chemotaxonomy is based on the chemical constituents of the plant.
Fig. algae, bryophytes, pteridophytes, gymnosperms, angiosperms
- Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic plants.
- They occur in moist stones, soils, and wood or in association with fungi and animals.
- Lichen is the association of algae with fungi.
- Algae grow on the body of sloth bear.
Fig. sloth bear
- Some algae are unicellular, some exist in colonial or filamentous forms, and a few marine plants form massive plant bodies.
- The algae reproduce by vegetative, asexual, and sexual methods.
- Vegetative reproduction is by fragmentation, where each fragment develops into a thallus.
- Asexual reproduction is by the production of flagellated zoospores which on germination give rise to new plants.
- Sexual reproduction takes place through fusion of two gametes, if the gametes with or without flagella are similar in size they are isogamous, if dissimilar in size they are anisogamous , and if female gamete is large, non-motile and male gamete is smaller, motile, the gametes are termed as
- Isogamous gametes with flagella are seen in Chlamydomonas and without flagella are seen in Spirogyra , anisogamous gametes are seen in in some species of Chlamydomonas, and oogamous gametes are seen in Volvox, Fucus.
- Algae are useful to human beings in the following ways:
- Fix carbon dioxide through photosynthesis and increase the level of dissolved oxygen.
- Primary producers of energy-rich compounds.
- Many species of Porphyra, Laminaria, and Sargassum are used as food.
- Certain marine brown and red algae produce large amounts of hydrocolloids which are used commercially.
- For example- align is produced by brown algae and carrageen is produced by red algae
- Agar, one of the commercial products obtained from Gelidium and Gracilaria are used to grow microbes and in preparations of ice-creams and jellies.
- Chlorella and Spirullina are unicellular algae, rich in proteins and are used as food supplements.
- The algae are divided into three main classes: Chlorophyceae, Phaeophyceae and Rhodophyceae.
- Commonly called as green algae.
- They are usually grass green due to the dominance of pigments chlorophyll a and b.
- Most of the members have one or more storage bodies called pyrenoids located in the chloroplasts, which contain protein besides starch.
- Green algae usually have a rigid cell wall made of an inner layer of cellulose and an outer layer of pectose.
- Vegetative reproduction usually takes place by fragmentation or by different types of spores, asexual reproduction is by flagellated zoospores produced in zoosporangia, and the sexual reproduction by the formation of sex cells and it may be isogamous, anisogamous or oogamous.
- Some commonly found green algae are: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.
Fig. Green algae
- Phaeophyceae or brown algae are found primarily in marine habitats.
- They possess chlorophyll a, c, carotenoids and xanthophylls.
- They vary in color from olive green to various shades of brown depending upon the amount of the xanthophyll pigment, fucoxanthin present in them.
- Food is stored as laminarin or mannitol.
- The vegetative cells have a cellulosic wall usually covered on the outside by a gelatinous coating of align.
- The plant body is usually attached to the substratum by a holdfast, and has a stalk, the stipe and leaf like photosynthetic organ – the frond.
- Vegetative reproduction takes place by fragmentation, asexual reproduction is by biflagellate zoospores that are pear-shaped and have two unequal laterally attached flagella.
- Sexual reproduction may be isogamous, anisogamous or oogamous where the gametes are pyriform and bear two laterally attached flagella.
- The common forms are Ectocarpus, Dictyota, Laminaria, Sargassum, and Fucus.
Fig. Brown algae
- Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body.
- The red thalli of most of the red algae are multicellular.
- The food is stored as floridean starch.
- The red algae usually reproduce vegetatively by fragmentation, reproduce asexually by non-motile spores, and sexually by non-motile gametes.
- The common members are: Polysiphonia, Porphyra, Gracilaria and Gelidium.
Fig. Red algae
- Bryophytes include various mosses and liverworts, commonly grow in moist shaded areas in the hills.
- Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction.
- Plant body is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids.
- They lack true roots, stem or leaves and may possess root-like, leaf-like or stem-like structures.
- The main plant body of the bryophyte is haploid, produces gametes, and hence is called a gametophyte.
- The male sex organ is called antheridium, which produce biflagellate antherozoids and the female sex organ called archegonium is flask-shaped, produces a single egg.
- The antherozoids are released into water where they come in contact with archegonium and fuses with the egg to produce the zygote.
- Zygotes produce a multicellular body called a sporophyte, which is not free-living but attached to the photosynthetic gametophyte and derives nourishment from it.
- Some cells of the sporophyte undergo reduction division to produce haploid spores, which germinate to produce gametophyte.
- The bryophytes are divided into liverworts and mosses.
- The liverworts grow usually in moist, shady habitats.
- The plant body of a liverwort is thalloid, the thallus is dorsiventral and closely appressed to the substrate.
- Asexual reproduction in liverworts takes place by fragmentation of thalli, or by the formation of specialized structures called
- Gemmae are green, multicellular, asexual buds, which develop in small receptacles called gemma cups located on the thalli.
- The sporophyte is differentiated into a foot, seta, and capsule.
- After meiosis, spores are produced within the capsule, the spores germinate to form free-living gametophytes.
- The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages:
- Protonema stage, which develops directly from a spore.
- Leafy stage, which develops from the secondary protonema as a lateral bud.
- They are attached to the soil through multicellular and branched rhizoids.
- Vegetative reproduction in mosses is by fragmentation and budding in the secondary protonema.
- In sexual reproduction, the sex organs antheridia and archegonia produce zygote which develops into a sporophyte, consisting of a foot, seta and capsule.
Examples of mosses are Funaria, Polytrichum.
- The pteridophytes are found in cool, damp, shady places.
- The main plant body is a sporophyte which is differentiated into true root, stem and leaves, possess well-differentiated vascular tissues.
- The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls, which may form distinct compact structures called strobili or cones.
- The sporangia produce spores by meiosis in spore mother cells, the spores germinate to give rise to thalloid gametophytes called
- The gametophytes bear male and female sex organs called antheridia and archegonia, respectively.
- Water is required for transfer of antherozoids to the mouth of archegonium and fusion of male gamete with the egg present in the archegonium result in the formation of zygote.
- Zygote produces a multicellular well-differentiated sporophyte which is the dominant phase of the pteridophytes.
- If all the spores are of similar kinds; such plants are called homosporous and if the spores are dissimilar such as micro and macrospores, these are called as
Example- Selaginella and Salvinia produce micro and macrospores.
- The development of the zygotes into young embryos take place within the female gametophytes.
- The pteridophytes are further classified into four classes
- Psilopsida (Eg. Psilotum)
Lycopsida (Eg.Selaginella, Lycopodium),
Sphenopsida (Eg.Equisetum) and
Pteropsida (Eg. Dryopteris, Pteris).
- The gymnosperms are plants in which the ovules are not enclosed by any ovary wall and remain exposed.
- The giant redwood tree Sequoia is one of the tallest tree species belongs to gymnosperms.
- The roots are generally tap roots, in some genera have fungal association in the form of mycorrhiza (Eg.,Pinus), while in some others (Eg.Cycas) small specialized roots called coralloid roots are associated with N2- fixing cyanobacteria.
- The stems are unbranched (Eg.,Cycas) or branched (Eg.,Pinus).
- The leaves may be simple or compound, such as pinnate leaves in Cycas and needle-like leaves in conifers.
- The gymnosperms are heterosporous, haploid microspores and megaspores are produced within sporangia on sporophylls which are arranged spirally along an axis to form compact strobili or cones.
- The strobili bearing microsporophylls and microsporangia are called microsporangiate or male strobili.
- The microspores develop into a male gametophyte which is highly reduced called as a pollen grain.
- The cones bearing megasporophylls with ovules or megasporangia are called macrosporangiate or female strobili.
- The megaspore mother cell is differentiated from one of the cells of the nucellus, protected by envelopes and the composite structure is called an ovule.
- The megaspore mother cell divides meiotically to form four megaspores, one of the megaspores enclosed within the megasporangium (nucellus) develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.
- The male and the female gametophytes and the pollen grain are released, carried in air currents and come in contact with the opening of the ovules borne on megasporophylls
- The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharge their contents near the mouth of the archegonia for fertilization which leads to formation zygote and finally uncovered seeds.
- In the angiosperms or flowering plants, the pollen grains and ovules are developed in specialized structures called flowers and the seeds are enclosed by fruits.
- They are divided into two classes
- The dicotyledons - characterized by having two cotyledons in their seeds.
- The monocotyledons- have only one cotyledon.
Fig. A dicot plant
Fig. A monocot plant
- The male sex organs in a flower is the stamen, consists of a slender filament with an anther at the tip and the anthers, following meiosis, produce pollen grains.
- The female sex organs in a flower is the pistil or the carpel, consists of an ovary enclosing one to many ovules.
- Within ovules highly reduced female gametophytes are present, termed as embryo sacs.
- The embryo-sac formation is preceded by meiosis.
- Each embryo-sac has a three-celled egg apparatus – one egg cell and two synergids, three antipodal cells and two polar nuclei, the polar nuclei eventually fuse to produce a diploid secondary nucleus.
- Pollen grain, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil this is termed as pollination.
- The pollen tubes enter the embryo-sac where two male gametes are discharged, one of the male gametes fuses with the egg cell to form a zygote (syngamy) and the other male gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus (PEN), termed as double fertilization.
- The zygote develops into an embryo and the PEN develops into endosperm which provides nourishment to the developing embryo.
- The synergids and antipodal cells degenerate after fertilization, during these events the ovules develop into seeds and the ovaries develop into fruit.
- The life cycle of an angiosperms is given below:
Alternation of Generation
- Haploid and diploid cells produce haploid and diploid plant bodies, respectively, by mitosis.
- The haploid plant body produces gametes and hence represents gametophytes.
- The zygote divides by mitosis to produce a diploid sporophytic plant body and this plant body produces haploid spores by meiosis, which again divides by mitosis to produce haploid plant body.
Different plant groups representing gametophytes and sporophytes differ in the following patterns:
- Sporophytic generation is represented only by the one-celled zygote and free-living sporophytes are absent.
- Haploid spores are formed by meiosis, which divide mitotically to form the gametophyte.
- The dominant, photosynthetic phase in such plants is the free-living gametophyte.
Examples-Volvox, Spirogyra and some species of Chlamydomomas.
Fig. haplontic life cycle
- Here, the diploid sporophyte is the dominant, photosynthetic, independent phase of the plant.
- The gametophytic phase is represented by the single to few-celled haploid gametophyte.
For example- gymnosperms and angiosperms.
Fig. diplontic life cycle
- A dominant, independent, photosynthetic, thalloid or erect phase is represented by a haploid gametophyte and it alternates with the short lived multicellular sporophyte. Example- bryophytes
- The diploid sporophyte is represented by a dominant, independent, photosynthetic, vascular plant body. It alternates with multicellular, saprophytic/autotrophic, independent but short-lived haploid gametophyte. Example- pteridophytes.
- Some alga genera such as Ectocarpus, Polysiphonia, kelps are haplo-diplontic.
Fig. haplo-diplontic life cycle